Les Gregor's map of common soils in the United States doesn't use pictures of dirt, but thin layers of real dirt. He wrote to the departments of agriculture in each state requesting a sample. Slate's Seth Stevenson described what happened next:

When I got in touch with Gregor, he explained that he asked each state for a "representative" soil. Many sent theirofficial state soils—which, wow, who knew that was a thing? Michigan sent two soils, so Gregor blended them together. Other soils came in clumps that he had to sift.

His favorite soils came from Colorado ("quite reddish"), Maine ("pale and sandy"), and Mississippi and Alabama ("deeply colored with iron oxide"). There are varying degrees of acidity. Every soil is a slightly different color. "It makes a nice quilt," says Gregor.